Go with a single hydro5. you can drive it with a battery powered air pump and it'll keep the 40 gal in great shape. Sylicone the base of the hydro to the bottom of the tank to prevent it moving around. Sometimes the simple solutions really are the best
You can pick them up at most better LFS's or order them on Ebay. It's a simple sponge filter thats amazingly effective. I'll post a link in a moment...
I also think a hydro size 5 is your best bet but I must caution you that keeping a strong biological filter going will be a challenge unless you constantly have a sizable volume of fish in the tank.